Electrode binder pitch



May 22, 1962 J. H. M NAMARA ET AL ELECTRODE BINDER PITCH 2 Sheets-Sheet1 Filed Jan. 27, 1961 mwwwoqzzwu ,0 LOW Tempemiu re Tar PiTch James H.McNamara Mario JI Caprio I- I7iKe A. Miller INVENTOR 95 x471:

ATTORNEY May 22, 1962 .1. H. MCNAMARA ET AL 3,035,932

ELECTRODE BINDER PITCH Filed Jan. 27, 1961 2 Sheets-Sheet 2 is j;

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e 6 CE I0 IO 6O [00 Weight "A Low Temperature Er Pitch 9- Resisttvitg(ohm-in.) x 10' I l I l l I l I I0 20 30 I0 50 (,0 70 90 90 /00 Weight/o Low Temperature Tar Pitch James H. M: Namara Mario 3'. Caprio i MiKeA. Miller INVENTORS ATTORNEY United States Patent Office 3,935,932Patented May 22, 1962 3,035,932 ELECTRODE BINDER PITCH James H.McYamara, Cheswick, and Mario J. Caprio and Mike A. Miller, NewKensington, Pa., assignors to Aluminum Company of America, Pittsburgh,Pa.,

a corporation of Pennsylvania Filed Jan. 27, 1961, Ser. No. 86,242 4Claims. (Cl. 106-484) This invention relates to the utilization of tarsproduced in the carbonization of bituminous materials. Moreparticularly, it is directed to an electrode binder comprised of alow-temperature tar pitch and a hightemperature tar pitch.

Peat, brown coal, lignite, sub-bituminous and bituminous coals arebituminous materials which have been proposed as feedstocks forlow-temperature carbonization processes to secure chars or cokes for useas fuel, and tars from which valuable products might be obtained. Thepresent invention is directed to the utilization of such low-temperaturetars in the production of electrode binder pitch.

The term low-temperature carbonization, as used herein, refers tocarbonization of bituminous materials at temperatures lower than about1300 F. Representative of such a process is that described by V. F.Parry in U.S. Patent 2,773,018 and in Drying and Carbonizing Fine Coalin Entrained and Fluidized State, Bureau of Mines Report ofInvestigations 4954, U.S. Department of Interior, dated April 1953.

The term low-temperature tar, as used herein, refers to tars produced bylow-temperature carbonization of peat, brown coal, lignite,sub-bituminous or bituminous coals. Such tars are generally oily, tarryorganic masses ranging from viscous liquids to soft semi-solid materialsat room temperature and may contain small quantities of char, ash orother inert material, dissolved gases and water.

The term low-temperature tar pitch, as used herein, refers to pitchobtained by distillation of low-temperature tars.

The term high-temperature tar, as used herein, refers to tars producedby carbonization of peat, brown coal, lignite, sub-bituminous orbituminous coals at temperatures above about 1600 F. The. most prevalentvariety are the coke-oven tars.

The term high-temperature tar pitch, as used herein,

refers to pitch obtained by distillation of high-temperature tars.

A typical analysis of low-temperature tar obtained by carbonization ofTexas lignite at 946 F. utilizing the Parry process, supra, is shown inTable 1 below.

TABLE 1 Analysis of Crude Tar 1 Method disclosed in article by G. U.Dlnneen et al., Shale Oil Naphthas: Analysis of Small Samples by SilicaGel Adsgpgrlcgn Method, Analytical Chemistry, vol. 19, p. 99

Low-temperature carbonization is generally favored for the production oflarge quantities of tar as compared to high-temperature processes, thuspermitting recovery of considerably greater amounts of tar oils.However, considerable differences exist in the nature of the tars.High-temperature tars are completely aromatic and contain only about 2to 10 percent of phenolic constituents, but substantially all of theseare low-boiling tar acids. Low-temperature tar is of lower specificgravity and is only partially aromatic, i.e. about l045 per cent; itcontains relatively large quantities of phenolic constituents, rangingfrom 20 to 40 percent of the tar. The pitches obtained by distillationof these tars are chemically similar to the initial feedstock, the lowermolecular Weight compounds having been removed. Further informationregarding the nature of these two tars can be found in Chemistry of CoalUtilization, vol. II, chapter 31, edited by H. H. Lowry, and Asphaltsand Allied Substances, by Herbert Abraham, vol. I, chapter XVII.

Generally, the product recovery treatment of lowtemperature tarsinvolves a distillation process to recover oils from which the tar acids.are removed. These phenolic constituents are usually a readilymarketable commodity and constitute a definite enhancement to theprocess. Tar bases may also be extracted from the distillate although oflesser commercial significance. The neutral oil remaining after suchextraction has been proposed as a fuel, or as a feedstock for variousrefining operations, or for separation into its constituent fractions.The pitch residue which constitutes a great percentage of the originaltar, about 25 to percent, has been considered of poor economic value andused as a fuel or briquetting binder. Much experimental Work has beendevoted to the development of more valuable use for this pitch,including the preparation of binders for carbon products.

Binder pitches for carbon electrodes, especially anodes for theelectrolytic production of aluminum, must meet certain criteria.Generally, they must produce electrodes having low resistivity andsatisfactory reactivity, and it has heretofore been considered thatthese properties could only be obtained by a high-temperature pitchbinder. Accordingly, pitches derived from high-temperature tars,generally coke-oven pitches, have been used in the production of theseanodes.

The usual procedure for the production of pre-baked carbon anodesconsists of mixing a high-temperature tar pitch (coke-oven) binder withthe coke or carbon aggregate (usually pctroleum coke) at a temperaturehigh enough so that the pitch binder is fluid. The green electrode mixis then molded into the desired shape under pressure, and then theplastic-shaped articles or green electrodes are baked at a slowly risingtemperature to an ultimate temperature above about 1000 C. so that thepitch binder is carbonized and the anode achieves the desired physicalstrength.

Because of the economics of low-temperature carbonization processes, itwould be greatly advantageous if low-temperature tar products could beused to produce anode binders. However, attempts to use pitches derivedfrom the distillation of low-temperature tars have resulted in totallyunsatisfactory anodes. Several processes have been developed forconversion of the low-temperature tar into a more aromatic substancefrom which satisfactory binders have been derived; however, theseconversion processes are costly.

It is an object of this invention to utilize low-temperature tars in theproduction of binder pitches for carbon electrodes.

It is also an object to provide an improved electrode binder pitchcomposition in which low-temperature tar pitch is employed.

Other objects and advantages will be obvious from the following detailedspecification and attached drawings wherein:

FIG. 1 is a photomicrograph illustrating the wetting power of variouspitch binders on petroleum coke;

FIG. 2 is a graph representing the viscosity of various pitch blends;

FIG. 3 is a graph showing the effect of the percentage oflow-temperature tar pitch on the reactivity of anodes made therewith;

FIG. 4 is a graph illustrating the variation in anode resistivityoccasioned by varying amounts of low-temperature tar pitch in the anodebinder.

It has now been found that a desirable binder pitch for carbonelectrodes can be prepared by admixing pitch prepared by distillation oflow-temperature tars with pitch obtained from high-temperature tars andwherein the lowtemperature material constitutes about to 90 percent byweight of the binder, and preferably about to 75 percent.

This pitch mixture has been found to effect greater wetting of the cokeor carbon aggregate. The properties of the anodes produced therefrom areequal and often superior to those of anodes prepared with the standardhigh-temperature or coke-oven pitch binders, especially with respect toreactivity.

By microscopic examination, it has been observed that the purelow-temperature tar pitches fail to wet the petroleum coke particles andthat the high-temperature or cokeoven pitch effects only a small amountof wetting. However, blends of the two materials in the hereindescribedrange are noted to penetrate into the coke pores and wet the cokesurface. As stated previously, the mere addition of solvents to reducethe viscosity of the pitches is not comparable since the wetting poweris not necessarily improved. Illustrating this improved wetting effectis the photomicrograph of FIG. 1 wherein three binder pitches have beenplaced upon petroleum coke which is at a temperature of 160 C. A is acoke-oven or high-temperature tar pitch of 110 C. softening point(cube-in-air); B is a low-temperature lignite pitch having a softeningpoint of 120 C.; C is a composite binder pitch of the present inventioncontaining percent of the low-temperature material and 50 percent of thehigh-temperature tar pitch. As is evident from the photomicrograph, thelow-temperature material (B) remains in a ball upon the surface of thepetroleum coke aggregate and has apparently not wet the coke. Thehigh-temperature pitch (A) remains in substantially the same conditionindicating little or no wetting of the coke. However, the compositepitch (C) of the present invention has substantially completelypenetrated into the pores of the coke.

The wetting effect is of importance since non-porous and/ orhigh-density anodes are desired to obtain low resistivities and lowreactivity. The pitch blend tends to penetrate deeply into the pores ofthe coke aggregate wherein it is subsequently carbonized during anodebaking.

The nature of the cooperation between the aromatic high-temperature tarpitch and the normally deleterious low-temperature material is notcompletely understood. Anodes made from binders containing above 50% ofthe low-temperature material exhibit progressively increasing heating ofwhich is continued until 190 C.

resistivity 1 and reactivity 2 as the proportion of low-temperaturematerial is increased, as shown by FIGS. 3 and 4, and as a resultbinders containing not more than of the low-temperature tar pitch arepreferred. However, as FIGS. 3 and 4 show, use of as little as 10% ofhightemperature material results in a sharp drop in reactivity andresistivity. Binders containing as much as of low-temperature pitch canbe used depending on the economics of pitch and power at a particularlocation. Binder pitches having less than 25 percent by weight oflow-temperature tar pitch produce anodes with higher reactivity, asshown in FIG. 3, and reduced wetting power. Also, the economic advantagein using the less costly low-temperature material is minimized.

It has been noted that the optimum in the blend formulation approximatesthe optimum in a viscosity curve, as shown in FIG. 2. To this extent,reduction in the viscosity of the pitch is of significance, although notcontrolling since attempts to duplicate the improved wetting power ofthe blend by adding various solvent fractions to the high-temperaturetar pitches were without success.

The pitch viscosity is related to the properties of anodes producedtherefrom. Anode reactivity reaches its optimum range at approximatelythe optimum in the viscosity curve, as is shown by FIG. 3, and theresistivity rapidly rises above about 60 percent, as is shown by FIG. 4.The test anodes were prepared according to usual commercial practicefrom petroleum coke and 18.5 percent of a binde containing variousamounts of lignite low-temperature tar pitch in admixture with thestandard cokeoven pitch.

Either whole low-temperature tar, or so-called single phase extractsthereof, such as the hexane-soluble portion, may be distilled to obtainthe pitch. Atmospheric batch type of distillation is preferablyemployed, although vacuum flash distillation may also be used. The pitchobtained should preferably have a softening point (cube-in-air) betweenabout 80 C. and C. for present commercial practices, although highersoftening points may also be used.

The optimum blend within the previously described range should hedetermined for each mixture of starting material since there is somevariance in the nature of the pitches generally available. As mentionedpreviously, the low point of the viscosity curve for the mixture isgenerally a good indication of the optimum composition.

The two pitch components are preferably admixed in p: (E) (A) (1 wherep: resistivity in ohm-inches E:average voltage drop across probes A:cross-sectional area of sample (sq. in.)

1: current passed through samples (amperes) D: distance between probes(inches) 2 The reactivity of anodes in a fluoride electrolyte bath forthe production of aluminum from alumina has been successfullyapproximated in the laboratory by a sulfate reactivity test. This testmeasures the loss in weight of a carbon specimen, 1 inch in diameter byinch, when immersed in molten sodium sulfate at 960 C. It is consideredan excellent meas ure of the reactivity of an anode in a bath for theproduction of aluminum by the Hall process.

The viscosity of a binder pitch can easily be determined by thefollowing method. Eight hundred grams of pitch are placed in a conta1nerand heated to C. pitch temperature at wh ch time a thermocouple, stirrerand the spindle of a rotating bob type viscometer are placed in thepitch, the

is reached with intermittent stirring to reduce air bubbles. At C., theheating 15 discontinued, the mixture stirred a definite amount ofstrokes, and the temperature and the torque on the viscometer read.After 4 5 minutes, the mixture is again stirred the same amount, thetemperature is read, and at exactly fivcminutes the viscometer is read;this procedure is repeated until the pitch reaches 100 C. From the dataobtained, a ziscosiity-temperature plot for the pitch tested may beobaine the fluid or molten state and may then be added to the cokeaggregate in either molten or solid state.

Although about 17.5 percent by weight of the hightemperature pitchbinder in the green electrode mix has been generally used in thefabrication of pro-baked anodes for the electrolytic production ofaluminum, it has been found that 18.5 percent by weight is preferablefor the blends of this invention, and amounts of 15 to 20% by weight canbe used.

In accordance with the present invention, a lignite lowtemperature tarwas distilled at atmospheric pressure to obtain a pitch having asoftening point of 112 C. It was then admixed with a commerciallyavailable cokeoven or high-temperature tar pitch having a softeningpoint of 110 C. Equal parts of each were used and the blending was inthe molten or fluid state to achieve uniformity.

The composite pitch binder was then added to petroleum coke aggregate inthe amount of 18.5 percent by weight and anodes were prepared inaccordance with usual commercial practice, the final baking temperaturebeing about 1084 C. Reference anodes utilizing a 100 percent coke-ovenpitch binder were similarly prepared. On evaluation, the propertiesshown in Table 2 were obtained.

TABLE 2 Anode Properties Baked Resis- Percent Apparent tlvity, PercentBinder Binder Density, ohms-in. Reactiv- -lcc. (an) ty (av.) (av.)

High-temperature (coke-oven) pitch, 110 0., S.P 17. 5 1.44 0.0028 13Low-temperature (lignite) tch, 112 C., S.P 17. 5 1. 37 0. 0032 28gbov-ltlemperature pitchnfinn 18.5 1.36 0.0034 21 ow-temperature pi e50% High-temperature pitch"- 5 42 0029 6 1 The anode is weighed dry,then soaked for 24 hours in water containing a few drops of a detergentsolution. It is suspended in water and weighed; then it is removed,dried with a towel and weighed while wet. The difierence between theweight in water and wet weight is determined and then this figure isdivided into the dry weight to give the baked apparent density.

In a further instance, low-temperature tar pitch having a softeningpoint of 117 C., a commercial high-temperature tar pitch having asoftening point of 110 C., and petroleum coke aggregate were mixedtogether thoroughly at 153 C. The total pitch comprised 83% by weightlow-temperature tar pitch and 17% high-temperature tar pitch, andconstituted 17% of the mix. Anodes were prepared from the mix inaccordance with usual commercial practice, and were baked at 1100 C.Reference anodes were similarly prepared in which a 100%high-temperature tar pitch binder, and the abovementioned petroleum cokeaggregate, were used. When the anodes were evaluated, those made withthe mixture of pitches showed resistivity only 7% higher than those madewith only high-temperature pitch as the binder, and there was nosubstantial diiference in reactivity. Such an increase in resistivitycan be tolerated economically in some localities depending on therelative cost of binder and power.

This application is a continuation-impart of our application Serial No.752,013, filed July 30, 1958, now abandoned.

Having thus described the invention, we claim:

1. A binder for carbon electrodes, consisting essentially of anadmixture of low-temperature tar pitch and hightemperature tar pitch andwherein said low-temperature tar pitch constitutes 25 to 90 percent byweight of the admixture.

2. A binder for carbon electrodes, consisting essentially of anadmixture of low-temperature tar pitch and high temperature tar pitchand wherein said low-temperature tar pitch constitutes 40 to percent byweight of the admixture.

3. A green electrode mix consisting essentially of a mixture of carbonaggregate and a binder consisting of low-temperature tar pitch andhigh-temperature tar pitch, said low-temperature tar pitch constituting25 to percent by weight of the binder.

4. A green electrode mix for anodes used in the electrolytic productionof aluminum, consisting essentially of a mixture of carbon aggregate and15 to 20 percent of a binder consisting of low-temperature tar pitch andhigh-temperature tar pitch, said low-temperature tar pitch constituting25 to 90 percent by weight of the binder.

No references cited.

1. A BINDER FOR CARBON ELECTRODES, CONSISTING ESSENTIALLY OF ANADMIXTURE OF LOW-TEMPERATURE TAR PITCH AND HIGHTEMPERATURE TAR PITCH ANDWHEREIN SAID LOW-TEMPERATURE TAR PITCH CONSTITUTES 25 TO 90 PERCENT BYWEIGHT OF THE ADMIXTURE.